Arsenic behavior across soil-water interfaces in paddy soils: Coupling, decoupling and speciation

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• The soil-water interface (SWI) in paddy soils features a sharp redox gradient crucial for arsenic (As) transformation and transport.
• Understanding the coupled or decoupled behavior of elements like iron (Fe), manganese (Mn), and As within this gradient is vital but lacks direct evidence.

Purpose of the Study:

• To investigate the mm-scale elemental distribution across the redox gradient at the SWI in diverse paddy soils.
• To provide direct evidence on the interactions and potential decoupling of Fe, Mn, and As in relation to soil redox conditions.

Main Methods:

• Utilized mm-scale elemental mapping to analyze Fe, As, Mn, and phosphorus (P) distribution in five paddy soil samples.
• Investigated As species transformation along the soil profile to understand redox-driven changes.

Main Results:

• A strong positive linear correlation between dissolved Fe, Mn, As, and P was observed in 4 out of 5 soils, indicating general elemental coupling.
• Decoupling of Fe, Mn, and As was identified in one soil, with distinct release patterns following the redox ladder.
• Arsenite oxidation and arsenate reduction dynamics were observed, with the arsenite to total As ratio stabilizing at 75.5% in certain soil depths.

Conclusions:

• Elemental interactions at the SWI redox gradient in paddy soils are complex, exhibiting both coupled and decoupled behaviors.
• The findings provide direct evidence of multi-element dynamics influenced by redox conditions at the soil-water interface.
• Understanding these interactions is key for managing arsenic contamination in paddy ecosystems.